1. Field of the Invention
The present invention relates to a spindle structure for a magnetic disk device, and particularly to the structure of a spindle that prevents galling of the contact surface during insertion of bearings.
2. Description of the Related Art
As shown in FIG. 1, which shows the plan view of the construction of an ordinary magnetic disk device, the construction of magnetic disk device 1 includes motor (hereinbelow referred to as a "spindle motor") 13 that rotates magnetic recording medium 2 around the spindle shaft as the central axis, magnetic head 4 that is positioned against the magnetic recording medium 2 and that reads and writes data, and positioner 5 that has a magnetic head at one end, is equipped with a drive structure, and that effects positioning to a prescribed track of magnetic recording medium 2.
The structure of the spindle of the above-described magnetic disk device includes a spindle shaft, two bearings secured to the spindle shaft, and a rotating portion upon which the magnetic recording medium is mounted and which is rotatably supported by these bearings (hereinbelow referred to as the "hub"). These two bearings are generally used to receive between them the thrust load, called "pre-load," that is parallel to the longitudinal direction of the spindle shaft, and this thrust load eliminates "play" between the spindle shaft and the bearings to ensure smooth and stable rotation of the magnetic recording medium.
Moreover, when fabricating the spindle construction, the assembly of these bearings is demanded of extremely high accuracy. The accuracy of assembly of these bearings directly affects the accuracy of rotation of the magnetic recording medium and therefore has a great effect on the performance of the magnetic disk device. Either a bonding method or a press-fit method is employed to secure the bearings, and high accuracy is similarly demanded of the surface to which the bearings are secured.
FIG. 2 is a sectional view of an conventional spindle motor of the prior art. As shown in FIG. 2, this spindle motor 13 is made up of: spindle shaft 6 secured in a housing; hub 17 and permanent magnet 10 which constitute a rotor that holds magnetic recording medium 2 (refer to FIG. 1); two bearings 18a and 18b that are secured to spindle shaft 6 and that rotatably support hub 17; and stator 9 that includes a coil.
When fabricating the above-described spindle motor 13, bearings 18a and 18b are secured to hub 17 by a bonding method or by a press-fit method. This process, however, tends to cause "galling" of the contact surfaces of bearings 18a and 18b and hub 17, and this galling presents a great impediment to ensuring the accurate rotation of spindle motor 13.
Other spindle constructions have been disclosed for preventing galling (such as in Japanese Patent Laid-open No. 155459/88) in which the outer circumferences of the spindle bearings are in a tapered form and the bearing are held in bearing housings arranged at both ends of the shaft.
To prevent the above-described galling, no effective measures have been available other than improving the processing accuracy of the insertion surface or improving the accuracy of jigs used in assembly. These methods of the prior art have not always been effective, and defects have occurred in fabrication. As a result, the insertion and securing of bearings during assembly of the bearings requires extra care to prevent minute galling of the surfaces to which the bearings are secured, and this need for extra attention inevitably results in reducing the yield in manufacturing magnetic disk devices.